This invention pertains to the art of control systems and more particularly to a controller for regulating fluid flow in a pressurized fluid system with attention to various time, pressure and safety parameters.
The invention is particularly applicable to a controller for gas and oil wells including a plunger lift device for maximizing production and efficiency by selectively regulating production and shut-in through close supervision of time, pressure and safety parameters. The production may be controlled by pressure only, time only, or any combination of the two. In addition, a plunger arrival sensor and delay time programming allows for selective operation to further maximize production and efficiency. However, it will be appreciated by those skilled in the art that the invention could be readily adapted for use in other environments as, for example, where similar control devices and systems are employed to control and regulate other types of fluid transmission and communication.
Gas and oil wells typically have varying production characteristics attributable to such well factors as depth, the types and quantity of fluids present in the well, and the natural gas "rock" pressure. Fluid accumulations in the well tubing particularly inhibit the gas production and, accordingly, should be removed. Such fluid accumulations are usually comprised of salt water and oil. Accordingly, dependent upon the particular characteristics of the well, differing well operating techniques are necessary to adapt to and handle these factors and thereby optimize production.
Some oil and gas wells are produced by using the plunger lift method by which a plunger lifts the liquids (e.g. oil, water) out of the well tubing by using the gas pressure in the casing of the well. The standard method of producing such wells is by a time "on" and time "off" cycle. An "on" period means that a designated time or pressure has been reached and a flow valve is opened which vents the pressure in the well tubing and allows the plunger to come to the top of the well bringing oil or water ahead of it. After the plunger has arrived at the top of the well, gas is allowed to flow out of the well through the well tubing. An "off" period means that the flow valve is closed, the plunger falls to the bottom of the well tubing and the well is sitting idle accumulating gas pressure, which will be used to move the plunger during the "on" cycle, and liquids.
Most wells employ a controller system which alternately shuts-in the well for pressure accumulation in the well casing and then opens it to allow the expelling of gas and fluids through a tubing received in the casing. The various forms and types of well controllers that have heretofore been suggested and employed in the industry have met with varying degrees of success. It has been found that the defects present in most prior well controllers are such that the controllers themselves are of limited economic and practical value.
To better understand the control functions and capabilities of the present invention, a comparison with previous controllers must be made. The present invention will not only perform all of the functions of all the controller types, but has incorporated additional functions to resolve their shortcomings. Functions beyond the capabilities of previous controllers have also been incorporated to provide production versatility not previously available to the producer.
One method of oil and gas production is by means of a simple open/close time cycle controller or variation thereof. An improvement over the simple timer is shown in U.S. Pat. No. 4,150,721 issued to Norwood. The Norwood controller can modify its preset production time cycle by inputs from manually set pressure switches located on the casing, tubing, or sales line and by limit switches which can indicate low flow rate, plunger arrival, or fluid storage full. The actuation of any of the limit switches performs only one of two functions; that is, the initiation of open time or the initiation of close time.
U.S. Pat. No. 4,355,365 issued to McCraken is similar to the Norwood controller. The open cycle time may be initiated by a high limit input from a manually set pressure limit switch on the casing. The close cycle time may be initiated from any low or off limit input from the manually set pressure limit switches on the casing, tubing, or sales line or by the switch indicating plunger arrival. The McCraken controller will also extend the open cycle time by the duration of a high limit input or extend the close cycle time by the duration of a low or off limit input to insure a full time count per cycle.
The manual pressure switches used by both Norwood and McCraken are of the Murphy switch type and may not be set to accurate pressure settings nor may the high and low limit contacts be set within 50 psi. The switches are subject to premature actuation due to pressure fluctations or vibrations during the production cycle. Neither the Norwood nor the McCraken controllers address the problem of premature termination of time cycle by fluctating pressures. During the open cycle, the pressure measured at the tubing and sales line may exceed preset limits due to the passage of the fluid slug. It would be desirable to use analog pressure transducers with an accuracy greater than 1 psi which provide for limits within 2 psi and are resistant to vibration. It would be desirable to prevent the premature shut-in of a well by providing a programmable limit delay time. If a pressure limit setting has been exceeded, the delay time will begin to time out. If at the end of the delay time, the pressure is again within the limit settings, no action will be taken. If at the end of the limit delay time the pressure still exceeds its limit, the well will shut in.
Another problem with previous time cycle production control systems is the failure of the controllers to provide a means for synchronization of several producing wells into a common sales line. This is more desirable with a field of low volume or stripper wells. A producing well with relatively low rock pressure will not be able to discharge its gas into a sales line which has greater pressure. The stronger wells must be shut-in to permit the weaker wells to produce. Timing of the various wells is important as fluid may build and load up the weaker wells. A time cycle controller must be used but must also have pressure limits and conditional inputs for safety. The Norwood and McCraken controllers have the pressure limits and conditional safety inputs but neither are suitable for synchronization for their time cycles are modified by the inputs. The McCraken controller extends its time cycle with the inputs and both McCraken and Norwood controllers terminate the open cycle and transfer the close cycle time low limit or off inputs.
It would be desirable to maintain a `constant time` with reference to the open and close cycles. All wells feeding the common sales line may then be assigned an operating period synchronized to within one second from any given time. This permits the individual wells the opportunity to produce at their optimum. The safety and line pressure limits would override an open cycle to close the main production valve only as long as the input condition exists. The time cycle would not be modified and the production valve would reopen for the duration of the open cycle when the line pressure limit or input condition no longer is active. If during the shut in condition of the open cycle, the open cycle time is complete, the close time would be transferred and the well would remain shut in.
It is desirable in colder climates to purge the remaining fluids from the production line upon completion of the fluid discharge cycle to prevent the accumulation of thickening or freezing solids. Previous time cycle controllers do not provide for this function. As shown in the Norwood and McCraken disclosures, when the control device is used with a plunger lift well, the open cycle is terminated upon arrival of the plunger and the production valve will immediately shut. The McCraken controller provides a means of switching out the plunger input and the plunger input may be removed from the Norwood controller to permit shut in on time out or low pressure limits. This will allow the production valve to remain open after the passage of the fluids for the continued sale of gas and incidentally purge the line. However, this method will greatly decrease the potential production for most wells.
It would be desirable to provide a programmable purge delay time. The function of the purge delay time would be to delay the closing of the main production valve after the passage of the fluid slug. During the purge delay time, which would be initiated with the arrival of the plunger, gas would be discharged through the production line. Upon time out of the purge delay time, the main production valve would be permitted to close. This is an override and does not affect any system times or cycles. If a safety or line pressure limit is active during the purge operation, the production valve may be temporarily closed until the shut-in condition ceases.
Most wells, even predominant gas wells, will produce a certain quantity of fluids. If the fluid is not expelled periodically, it will eventually load up and restrict the gas production. A time cycle controller may be improved to include a means for the production of gas wells independent of absolute cycle times. The Norwood and McCraken controllers both provide an input for high casing pressure turn-on. The controllers are usually used with a plunger lift system to provide efficient removal of the fluid content. A problem occurs, however, with the time cycle controllers when the rock (casing) pressure of the well remains high for an extended period of time. Upon the plunger arrival during the open cycle, the controllers initiate the close cycle and begin to close the production valve. If the casing pressure is still above the limit, the controller will immediately try to initiate the open cycle. The plunger will therefore oscillate near the top of the tubing as the production valve alternately toggles between open and close. The actual operating speed of the production valve is quite slow compared to the alternate commands it is receiving and therefore will remain open until the high casing pressure falls below the preset low limit. This may permit an excessive amount of fluid to enter the tubing, which in turn, restricts the discharge of gas and therefore maintains the high casing pressure. Raising the pressure limit on the casing switch may correct the problem, but it will also reduce the production of gas and possibly create a very dangerous situation if the plunger is driven hard with little fluids into the receiving mechanism at the surface.
The previous time cycle controllers would also remove the plunger arrival input from the circuit to prevent premature termination of the gas sales upon the plunger arrival. Failure to monitor the plunger arrival could have serious consequences. The plunger provides a partial seal between gas and fluid, reduces fluid fallback, and more efficiently uses gas lift energy; however, the plunger is also used to remove paraffin and scale from the tubing walls and may occasionally hang up. An expensive swabbing operation may then be required to remove the fluids after freeing the plunger if the available casing pressure is insufficient to discharge the accumulated fluids.
To address this problem, it would be desirable to use a special delay in the time cycle mode of operation or to operate in the full differential controller mode. A programmable plunger delay would be provided for use in the time cycle control mode to insure the return of the plunger to the bottom of the tubing after arrival with the fluid slug. This delay would not be overriden by any open pressure limit or input until time out. If the casing pressure remains high for an extended time, the purge delay time would be set to provide sufficient additional gas sales after the plunger arrival. The plunger delay would be set to insure the return of the plunger to the bottom of the tubing for removal of any accumulated fluids. A programmable shut in time would also be provided to monitor the plunger arrival within a specific time period and prevent the accumulation of fluids if the plunger hangs-up.
A predominant gas well may be better produced with a differential type controller which initiates the production cycle on pressure and fluid accumulation. U.S. Pat. No. 3,266,574 issued to Gandy is an early attempt to produce gas wells which have very little fluid content. The method of production was an improvement over the time cycle method at the time because the fluids were discharged directly through a separate fluid production valve into fluid storage tanks. If the fluid production valve remained open after the passage of the fluid slug, then gas was discharged into the atmosphere for the remainder of the open time cycle. The Gandy device was always used with a time cycle controller and would inhibit the opening of the fluid production valve until the pressure differential between the casing and tubing indicated sufficient fluids were accumulated to permit expulsion during the time cycle controller's open cycle. The production gas was fed directly into the sales line but was restricted to prevent entry of fluids by natural flow.
U.S. Pat. No. 3,863,714 issued to Watson is an improved differential controller which uses only one production valve to discharge both the gas and fluid components. The controller will open the production valve when the tubing pressure is sufficient to discharge both gas and fluid accumulations into the sales line. The flow rate between the tubing and sales line is then monitored. A decrease in the flow rate will indicate an accumulation of fluids or the depletion of available rock pressure. When the flow rate drops below the preset value of the flow switch, the controller will close the production valve until tubing pressure has again increased sufficiently to initiate the restart of the cycle. The controller makes no provisions for conditional operation as is required with plunger lift wells.
U.S. Pat. No. 4,526,228 issued to Wynn is an improved version of the differential controller which has been enhanced to modify the production cycle with time and pressure limits. It also incorporates inputs for plunger arrival and safety limits. The Wynn controller uses a gas production valve connected to an oil and gas separator and fluid production valve directly connected to a fluid storage tank. If the casing pressure increases sufficiently to feed gas into the sales line, the gas discharge valve will open. When the differential pressure between the casing and tubing indicates an accumulation of fluid in the tubing and sufficient casing pressure is available to discharge the fluid, the fluid production valve will open. A problem will occur with the Wynn controller if the discharge of gas into the sales line has been great enough to permit the plunger to raise to the surface before the opening of the fluid discharge valve. The plunger will not have sufficient time to fall below the fluids as the Wynn controller has no delay time between the closing of the gas discharge valve and the opening of the fluid discharge valve except for a limited valve speed adjustment.
It would be desirable to provide a programmable delay between the closing of the main production valve and the opening of the fluid bypass valve to insure plunger `fallback`. The arrival of the plunger at the top of the tubing, in the Wynn controller, indicates that the fluid has been expelled and initiates closure of the fluid discharge valve and transfer of a downtime period. The fluid discharge valve may not be reopened until time out of the downtime period. If the discharge of the gas causes too great a pressure differential, indicating a maximum allowable fluid build up, or if the casing pressure falls below a preset limit, the gas discharge valve will close. The fluid discharge valve will also remain closed if the tubing pressure exceeds a maximum safe pressure for expulsion into the fluid storage tank. If during the open fluid discharge cycle, the plunger fails to arrive at the top of the tubing within a predetermined time, the fluid and gas discharge valves are both shut-in until the pressure differential between the casing and tubing again falls within the maximum allowable setting. A problem will occur with this production method if the plunger hangs up. When the plunger fails to arrive within the predetermined time during the open fluid cycle, the gas discharge and fluid valves are closed until the pressure equalizes through the fluid at which time the cycle is again initiated. Although the gas discharge valve will shortly close as the differential pressure exceeds the maximum, the fluid discharge valve will remain open for the duration of the predetermined uptime. As the tubing pressure will be lowered to atmospheric through the fluid discharge line, casing pressure will escape around the plunger and through the fluid to reduce the back pressure on the oil bearing formation and permit additional fluid accumulations. The well may soon load up beyond its capacity to discharge the fluids as the cycles continue. It would be desirable to prevent the accumulation of fluids using a programmable shut-in time to monitor the arrival of the plunger and initiate a total shut-in upon failure.
Another production problem occurs when the gas sales line pressure remains higher than the well's rock pressure for an extended period or when the well is shut-in to limit its gas production to the maximum allowable by the purchaser. It is most desirable to continue to produce oil under these conditions rather than lose both oil and gas sales. Although the Wynn controller incorporates time limits and provides for safety shut-in overriding, it must see a differential pressure between the casing and tubing to initiate the fluid valve opening. This differential is the result of fluids restricting the flow of gas from the casing, through the tubing, and into the gas sales line. If gas is not permitted to flow into the sales line, no differential pressure will be created and thus, the controller can not expel its fluids. The time cycle controllers are likewise limited under these conditions because gas flow is required through the production valve to provide enough pressure differential to raise the fluids.
It would be desirable to overcome this type of production problem by combining both time cycle functions and differential controller operations. A typical sequence of operation would be initiated when the casing pressure has increased to a programmed value indicating sufficient pressure to expel fluids. The main production valve, which feeds both oil and gas into a separator, opens and a maximum open time period is transferred to the systems clock. The plunger will normally soon arrive with the fluid slug and gas will continue to be discharged into the sales line until the accumulation of fluids in the tubing increases the casing/tubing differential pressure to its programmed limit. The main production valve is then shut-in and a plunger delay time is initiated to insure that the plunger has sufficient time to drop through the fluid to the bottom of the tubing before the production valve is reopened and the fluids discharged. In the event that the gas flow from the production valve is inhibited, a secondary sequence is initiated. The main production valve will be opened and maximum open time transferred as before upon sufficient casing pressure and at the end of any plunger delay time. The absence of sufficient gas flow to raise the plunger will cause the production valve to be closed at the completion of the maximum open time and the auxiliary bypass valve, which discharges directly into the fluid storage tank will be opened. The plunger will now arrive at the top of the tubing with the last of the fluid and cause the bypass valve to immediately close. The sequence then returns to normal with the transfer of plunger delay time. It would also be desirable to enter various safety pressure limits and time limits for more complete control.
The inclusion of an auxiliary bypass valve would also be desirable since it could serve as a fluid bypass in the time cycle mode of operation permitting production of oil and gas wells having low natural rock pressure. Such a bypass valve could also serve as the main production valve for the injection gas lift method of production.
U.S. Pat. No. 4,410,038 issued to Drapp describes an intermittent well controller which uses a main valve to supply additional pressure to the casing of a plunger lift well which has little natural rock pressure. The operation of the Drapp controller is basically that of a time cycle controller which will open a supply valve periodically to permit the injection of gas into the casing. The pressure of the injected gas is such that it will drive the plunger and the fluids to the surface. Upon the arrival of the plunger, the supply valve is shut-in and the plunger then drops again to the bottom of the tubing. The cycle may not be restarted until the completion of the selected cycle time.
Although the Drapp controller is based upon a type of low power microcomputer, its operator entry functions are severely limited to manual override and selection of number of cycles in a 24 hour period. The functions of the pressure limit switches are vague except for the maximum high casing pressure limit which will terminate the open supply valve cycle.
A major shortcoming with the Drapp controller is the assumption of a consistent high pressure source for injection upon initiation of the cycle and failure to provide cycle time modification by conditional inputs. If the available injection pressure source varies and causes only partial expulsion of the fluids or is insufficient for expulsion, loading of the well will occur due to continued fluid accumulation. The Drapp controller has no means of preventing continued fluid accumulations as it has no production control valve to restrict loss of tubing pressure. It would be desirable to use an injection supply valve and a production valve to prevent the accumulation of fluids by casing/tubing pressure equilization when insufficient injection source pressure is available.
The present invention contemplates a new and improved controller which overcomes all of the above referred to problems and others and meets the above stated needs to provide a new gas and oil well controller which is readily adaptable to a plurality of well operational uses with wells having a variety of operational characteristics and parameters, and which is easy to install, easy to operate, and which provides optimally improved well control and production.